This invention relates to additives for improving the processability of molten polymer compositions wherein the major component is a halogen-containing polymer. This invention further relates to additives that improve the appearance and physical properties of articles formed from halogen-containing polymer compositions.
It is well known to employ various additives to modify the properties of halogen-containing polymers, particularly homopolymers and copolymers of vinyl chloride. Among the most frequently employed additives are heat stabilizers, lubricants such as paraffin waxes and esters or salts of fatty acids to improve the flow properties of the composition and processing aids such as poly-.alpha.-methylstyrene and polymethyl methacrylate to increase the melt strength of the polymer. Melt strength is of particular importance for extrudable compositions, since it is a prime factor in determining the maximum extent to which the diameter of a continuous solid extrudate or the wall thickness of a continuous hollow extrudate such as a pipe can be reduced uniformly in the absence of breaks and other defects. The melt strength of a polymer formulation is also a major factor in determining the ease with which the initial portion of an extrudate formed from the formulation can be fed by hand through vacuum sizing tanks, rolls and other equipment required to shape or otherwise process the polymer formulation before it has completely solidified. This procedure, referred to in the trade as "stringing up" an extruder, is a required step in many extrusion processes, particularly if the product is a hollow object such as a tube or pipe.
The use of processing aids is of particular importance for the extrusion of pipe from vinyl chloride polymer compositions. The resultant higher melt strength enables the molten pipe to be drawn down to relatively small wall thicknesses. The range of wall thicknesses that can be produced without the necessity of changing the extruder die is significantly increased by incorporating one of the known processing aids in the polymer formulation. The use of these processing aids is not without some disadvantages, however. In addition to significantly increasing the cost of the polymer formulation, the presence of these processing aids imparts a brittleness and high gloss level to the final pipe.
Oxidized polyolefins having melt viscosities of less than 300 centipoises and a dicarboxylic acid content of from 20 to 80 weight % have been used to impart transparency to vinyl chloride polymers. U.S. Pat. No. 3,979,347 teaches that this property is characteristic of oxidation products of ethylene polymers wherein the polymers exhibit a molecular weight of from 10,000 to 1,000,000. Some of these polymers exhibit melt viscosities of from 1,000 to 60,000 centipoises at 140.degree. C. The oxidized polymer is the only lubricant present in the composition. These compositions are not suitable for extrusion because of their high melt viscosities. The large amount of torque required to process such a composition is above the operable limit for most extruders. The amount of electrical current required to achieve this torque could damage the motor of the extruder. The addition of a paraffin wax or other conventional lubricant for the purpose of lowering the melt viscosity would decrease the transparency of the final article, which is contrary to the objectives of the aforementioned U.S. Pat. No. 3,979,347.
It is an objective of this invention to increase the melt strength of compositions containing polyvinyl chloride or other halogen-containing polymers without imparting undesirable properties to the final shaped article, particularly the high gloss and brittleness that are characteristic of conventional processing aids.
It has now been found that this objective can be achieved using polymer compositions containing polyvinyl chloride or other halogen-containing polymer, a conventional external lubricant, such as a natural or synthetic paraffin wax, a conventional heat stabilizer and an oxidized ethylene homopolymer or an oxidized copolymer of ethylene and at least one additional .alpha.-olefin, with the proviso that the oxidized polymer exhibits an acid number greater than about 5 and a melt viscosity of from 1,000 to about 60,000 centipoises at a temperature of 140.degree. C.